1. Field of the Invention
Embodiments of the invention relate to light emitting diodes (LEDs) and, more particularly, to white LEDs utilizing wavelength conversion materials.
2. Description of the Related Art
A light emitting diode (LED) is a solid state device that converts electric energy to light and generally comprises one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.
Conventional LEDs cannot generate white light from their active layers. Light from a blue emitting LED has been converted to white light by surrounding the LED with a yellow phosphor, polymer, or dye, with a typical phosphor being cerium-doped yttrium aluminum garnet (Ce:YAG). [See Nichia Corp. white LED, Part No. NSPW300BS, NSPW312BS, etc.; Cree® Inc. EZBright™ LEDs, XThin™ LEDs, etc.; See also U.S. Pat. No. 5,959,316 to Lowrey, “Multiple Encapsulation of Phosphor-LED Devices”]. The surrounding phosphor material converts the wavelength of some of the LED's blue light, changing its color. Some of the blue light passes through the phosphor without being changed while a substantial portion of the light is converted. Depending on the phosphor properties the blue light can be converted to green, yellow, orange, or red light, or a combination thereof to create white light. In another approach light from a violet or ultraviolet emitting LED has been converted to white light by surrounding the LED with multicolor phosphors or dyes.
The spectrum of the light emitted by an LED depends on the spectrum (or wavelength) of the exciting (e.g. blue LED) light source, the amount of light converted by the phosphor, and the emission spectrum of the phosphor. Phosphors have excitation and absorption characteristics that vary with the wavelength of the exciting light source. Thus, the overall color of light emitted by a LED depends on many factors that include the LED wavelength, the phosphor mechanical and optical properties, and the phosphor layer thickness. This provides a great deal of flexibility in tailoring the output of LEDs but it also requires careful control of many parameters to insure a particular color.
The human eye is relatively sensitive to variations in emission wavelengths and can detect relatively small differences in emission wavelengths or color. Perceptible variations in color emitted by LEDs designed to emit a single color of light can reduce customer satisfaction and reduce overall acceptance of LEDs for commercial uses. In an effort to provide LEDs that emit light of the same or similar wavelength, the LEDs can be tested and sorted by color or brightness. This process is generally known in the art as binning. Each bin typically contains LEDs from one color and brightness group and is typically identified by a bin code. White emitting LEDs can be sorted by chromaticity (color) and luminous flux (brightness). Color LEDs can be sorted by dominant wavelength, luminous flux, or, in the case of certain colors such as royal blue, by radiant flux. LEDs can be shipped, for example, on reels containing LEDs from one bin and labeled with the appropriate bin code.
FIG. 3A shows one example of binning chromaticity regions plotted on the 1931 CIE Coordinate system for commercially available white emitting Cree® XLamp® XR-E and XR-C LEDs provided by Cree, Inc. Each of these regions corresponds to a particular chromaticity range of white LEDs with the regions shown surrounding the black body curve or black body locus (BBL). Each of these regions is designed to designate chromaticity variations that are within acceptable ranges for a given application.
FIG. 3B shows an example of how LED color can be affected by the wavelength of the blue LED used to excite the phosphor. In this case, white-emitting LEDs are fabricated by combining blue EZBright LED chips with a yellow-emitting phosphor coat, and then encapsulating them in a clear silicone lens. The emission wavelength of the LED chips is intentionally varied from 450 nm to 465 nm. The phosphor coating is the same for all devices. The chromaticity of the combined light is observed to vary across a number of color bins depending on the LED chip wavelength, due to the inherent wavelength-dependent phosphor properties. Achieving a particular chromaticity for all blue wavelengths therefore requires tailoring one or more phosphor properties for each blue wavelength.
LEDs and phosphors can be integrated at the LED wafer level, chip level, or at the package level. There has been recent interest in coating LEDs with a phosphor at the wafer level to reduce the cost and complexity of fabrication. See, for example, U.S. patent application Ser. Nos. 11/656,759 and 11/899,790 to Chitnis, et al., both of which are commonly owned with the present application and incorporated by reference herein. LEDs across a wafer can also have different emission characteristics or color spread. FIG. 1 shows one example of a wavelength emission map 10 for a wafer of blue emitting LEDs showing wavelength variations across the wafer, and each wafer can have its own unique emission map. In the map shown, the wavelength distribution is approximately 450 to 460 nm, although other wafers can experience different distributions in different wafer areas. This distribution can result from different factors such as variations in the epitaxial material during growth of the LEDs, or from variations in the flatness (i.e., bow) of the growth substrate.
The wafer can be coated with a conversion material (e.g., phosphor). FIG. 2 shows a conversion material thickness map 20 following coating. In some fabrication processes the coating can be planarized using known methods. The thickness of the coating can vary across the wafer due to different factors such as variations in the thickness of the underlying wafer and in planarization variations. In the embodiment shown, the wafer experiences a total thickness variation of approximately 3 μm. The wavelength emission variations of the LEDs and thickness variations of the conversion material across the wafer can result in a spread of emission wavelengths or color points of the LED chips singulated from the wafer, similar to the results shown in FIG. 3B. This spread can be exacerbated by phosphor loading or concentration variations across the wafer.